A method for the manufacture of reduced graphene oxide from electrode graphite scrap

ABSTRACT

A method for the manufacture of reduced graphene oxide from electrode graphite scrap including the provision of the graphite electrode scrap, the grinding of electrode graphite scrap to obtain ground graphite electrode, an oxidation step of the ground graphite electrode to obtain graphene oxide and the reduction into reduced graphene oxide.

The present invention relates to a method for the manufacture of reduced graphene oxide from electrode graphite scrap. In particular, graphene oxide will have applications in metal industries including steel, aluminum, stainless steel, copper, iron, copper alloys, titanium, cobalt, metal composite, nickel industries, for example as coating or as a cooling reagent.

BACKGROUND

Graphite electrode scrap is a residue in the steelmaking process. Indeed, Graphite electrodes scrap are used in electric arc furnaces (EAF). EAF is a furnace comprising the graphite electrodes that heats charged material, i.e. scraps, by means of an electric arc. After charging the EAF with scraps, the electrodes are lowered onto the scrap, an arc is struck and the electrodes are then set to bore into the layer of shred at the top of the furnace. Once the electrodes have reached the heavy melt at the base of the furnace and the arcs are shielded by the scrap, the voltage can be increased and the electrodes raised slightly, lengthening the arcs and increasing power to the melt.

The electrodes can be used several times but their life time is short. Usually, waste graphite electrodes scrap are open-air stored. It is known to use the waste graphite electrodes are used to synthesize Graphene. Graphene is a single layer of Graphite consisting of carbons that are bonded together in a hexagonal honeycomb lattice. In other terms, it is an allotrope of carbon in the structure of a plane of sp² bonded atoms.

The patent CN107673338 discloses a mechanical Exfoliation process. In this patent, the method for preparing graphene using a waste graphite electrode comprises following steps:

-   -   step 1: pretreatment, purification and liquid phase stripping         strengthening of the waste graphite electrode:     -   step 2: centrifugal classification and     -   step 3: post treatment.

However, the method for the manufacture of graphene comprises a multitude of steps. In particular, the pretreatment, purification and liquid phase stripping strengthening of the waste graphite electrode (step 1) includes a multitude of sub-steps extending significantly the method duration. Indeed, the method pulverizing the surface layer into graphite powder with a particle size of 20 μm to 60 μm, and placing the graphite powder in a liquid phase purification stripping solution containing an aluminum ion salt formulated and added in a mass ratio of aluminum element to graphite powder of 1:10 to 1:50 to perform purification and stripping strengthening treatment, controlling the pH of the liquid phase purification stripping solution at 0.5 to 2 during the treatment process, after finishing the treatment, allowing the solution to stand and stratify, resulting in the upper layer which is a graphene-containing cloudy solution and the lower layer which is coarse graphite and insoluble impurity particles, and separating the upper layer and the lower layer to obtain the graphene-containing cloudy solution ready for use; to prepare graphene. Then, a centrifugation is performed. Finally, an intercalation step is performed using sodium, aluminum and fluorine as an intercalation reagent to intercalate the graphite. Thus, Graphite can be exfoliated to obtain graphene. This method, especially the pretreatment, purification and liquid phase stripping strengthening step, is very difficult to handle at industrial scale. Moreover, by applying this method, it is only possible to obtain graphene, no other materials such as reduced graphene oxide since these compounds are completely different. The methods for producing graphene cannot be transposed to the method for manufacturing reduced graphene oxide.

Reduced graphene oxide is composed of one or a few layers of graphene sheets containing some oxygen functional groups. Thanks to its interesting properties such as a high thermal conductivity and a high electrical conductivity, reduced graphene oxide, being hydrophobic, has many applications as mentioned above.

It is known to produce reduced graphene oxide from Kish graphite. Kish graphite comprises a high amount of carbon, usually above 50% by weight, it is a good candidate to produce graphene based materials.

The patent KR101109961 discloses a method of manufacturing reduced graphene oxide, comprising:

-   -   a step of pretreating Kish graphite,     -   a step of manufacturing graphite oxide by oxidizing the         pretreated Kish graphite with an acid solution;     -   a step of manufacturing graphene oxide by exfoliating the         graphite oxide and     -   a step of manufacturing reduced graphene oxide by reducing the         graphene oxide with a reducing agent.

In this Korean patent, the pre-treatment of Kish graphite comprises: a flushing process, a process of purification using a chemical pretreatment composition and a mechanical separation process (separation by size). After the process of purification, the purified Kish graphite is separated by size, the Kish graphite having a particle size of 40 mesh or less, i.e. 420 μm or less, is kept for the manufacture of graphene oxide.

However, the pretreatment of Kish graphite comprises 2 steps using a chemical composition: the flushing step and the process of purification step. In the Example of KR101109961, the flushing step is performed with an aqueous solution comprising water, hydrochloric acid and nitric acid. Then, the process of purification is performed with a pretreatment composition comprising a chelating agent, an iron oxide remover, a surfactant, an anionic and nonionic polymer dispersant and distilled water. At industrial scale, two chemical treatments are difficult to manage since a lot of chemical waste has to be treated and the stability of such composition is difficult to control.

SUMMARY OF THE INVENTION

Moreover, the pretreatment composition of the KR101109961 reference needs a long time preparation. For example, in the Example, the reduction of graphene oxide into reduced graphene oxide is very long since it is performed during 24 hours. The productivity is therefore slowed. Additionally, the pre-treatment of Kish graphite including the process of purification using the pretreatment composition is not environmentally friendly.

Thus, there is a need to produce reduced graphene oxide by an environmentally friendly method in a short time to increase the productivity compared to conventional methods.

It is an object of the present invention to provide a less polluting method for the manufacture of reduced graphene oxide compared to the conventional methods. Additionally, it is an the object is to provide an industrial method to obtain graphene oxide having good quality in the shortest time possible.

The present invention provides a method for the manufacture of reduced graphene oxide from graphite electrode scrap comprising:

-   -   A. the provision of the graphite electrode scrap,     -   B. the grinding of electrode graphite scrap to obtain ground         graphite electrode,     -   C. an oxidation step of the ground graphite electrode to obtain         graphene oxide comprising the following successive sub-steps:         -   i. the preparation of a mixture comprising the ground             graphite electrode, an acid and a nitrate salt, the mixture             being kept at a temperature below 5° C.,         -   ii. the addition of an oxidizing agent into the mixture             obtained in step C.i) to obtain graphite oxide,         -   iii. after the targeted level of oxidation is reached, the             addition of a chemical element to stop the oxidation             reaction,         -   iv. the separation of graphite oxide from the mixture             obtained in step C.iii),         -   v. the exfoliation of graphite oxide into graphene oxide and     -   D. a reduction of graphene oxide into reduced graphene oxide.

The following terms are defined:

-   -   graphene oxide means one or a few layer(s) of graphene         comprising at least 25% by weight of oxygen functional groups,     -   reduced graphene oxide means graphene oxide that has been         reduced. The reduced graphene oxide comprises one or a few         layer(s) of graphene having some oxygen functional groups         and—Oxygen functional groups means ketone groups, carboxyl         groups, epoxy groups and hydroxyl groups.

Other characteristics and advantages of the invention will become apparent from the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate the invention, various embodiments and trials of non-limiting examples will be described, particularly with reference to the following Figures:

FIG. 1 illustrates an example of one layer of reduced graphene oxide according to the present invention.

FIG. 2 illustrates an example of a few layers of reduced graphene oxide according to the present invention.

DETAILED DESCRIPTION

The present invention relates to a method for the manufacture of reduced graphene oxide from graphite electrode scrap comprising:

-   -   A. the provision of the graphite electrode scrap,     -   B. the grinding of electrode graphite scrap to obtain ground         graphite electrode,     -   C. an oxidation step of the ground graphite electrode to obtain         graphene oxide comprising the following successive sub-steps:         -   i. the preparation of a mixture comprising the ground             graphite electrode, an acid and a nitrate salt, the mixture             being kept at a temperature below 5° C.,         -   ii. the addition of an oxidizing agent into the mixture             obtained in step C.i) to obtain graphite oxide,         -   iii. after the targeted level of oxidation is reached, the             addition of a chemical element to stop the oxidation             reaction,         -   iv. the separation of graphite oxide from the mixture             obtained in step C.iii) and         -   v. the exfoliation of graphite oxide into graphene oxide and     -   D. a reduction of graphene oxide into reduced graphene oxide.

Without willing to be bound by any theory, it seems that with the method according to the present invention, it is possible to produce reduced graphene oxide from graphite electrode scrap without a long time preparation step of the graphite electrode. Indeed, the grinding (step B) followed by the chemical oxidation of the electrode and the reduction of graphene oxide allows for a more environmentally friendly method compared to the method of the prior art in a short time increasing the productivity of reduced graphene oxide.

Preferably, in step B), the grinding is performed to obtain ground graphite electrode having a size less than 200 μm, more preferably less than 150 μm and advantageously between 100 and 150 μm. Without willing to be bound by any theory, it is believed that when the ground graphite electrode has the above size, the method productivity is further improved since the ground graphite electrode is completely oxidize in a shorter time. Indeed, it seems that the ground graphite electrode size can also have an impact on the method productivity, especially on the oxidation time, because the oxidizing agent can easily navigate between the graphite having the above size. So to obtain graphene oxide having a high oxygen percentage, there is a risk to increase the oxidation time when the electrode graphite size is outside the above range.

Preferably, in step C.i), the salt nitrate is chosen from: NaNO₃, NH₄NO₃, KNO₃, Ni(NO₃)₂, Cu(NO₃)₂, Zn(NO₃)₂, Al(NO₃)₃ or a mixture thereof. More preferably, the salt nitrate is chosen from NaNO₃ and NH₄NO₃.

Advantageously, in step C.i), the acid is chosen from: H₂SO₄, HCl, HNO₃, H₃PO₄, C₂H₂Cl₂O₂ (dichloroacetic acid), HSO₂OH (alkylsulfonic acid) or a mixture thereof.

Preferably in step C.ii), the oxidizing agent is chosen from: potassium permanganate (KMnO₄), H₂O₂, O₃, H₂S₂O₈, H₂SO₅, KNO₃, NaClO or a mixture thereof. In a preferred embodiment, the oxidizing agent is potassium permanganate.

Preferably, the reaction is realized at a temperature below 50° C. or at room temperature.

Then, in step C.iii), when the targeted level of oxidation is reached, a chemical element is added to stop the oxidation. The targeted level of oxidation depends on the oxidation degree of graphene oxide, i.e. having at least 45% by weight of oxygen groups according to the present invention. The level of oxidation of graphene oxide can be analyzed by scanning electron microscopy (SEM), X ray diffraction spectroscopy (XRD), Transmission electron microscopy (TEM), LECO analysis and/or Raman spectroscopy over time during the oxidation.

Then, advantageously in step C.iii), the chemical element used to stop the oxidation reaction is chosen from: an acid, non-deionized water, deionized water, H₂O₂ or a mixture thereof.

In a preferred embodiment, when at least two elements are used to stop the reaction, they are used successively or simultaneously. Preferably, deionized water is used to stop the reaction and then H₂O₂ is used to eliminate the rest of the oxidizing agent. In another preferred embodiment, H₂O₂ is used to stop the reaction and eliminate the rest of the oxidizing agent. In another preferred embodiment, H₂O₂ is used to stop the reaction by this following reaction:

2KMnO₄+3H₂O₂=2MnO₂+3O₂+2KOH+2H₂O.

Then, to eliminate MnO₂, an acid can be used. For example, HCl is added to the mixture so that the following reaction happens:

MnO₂+2HCl=MnCl₂ (soluble in water)+H₂O.

Without willing to be bound by any theory, it seems that when the element to stop the reaction is added into the mixture, there is a risk that this addition is too exothermic resulting in explosion or splashing. Thus, preferably in step C.iii), the element used to stop the reaction is slowly added into the mixture obtained in step C.ii). More preferably, the mixture obtained in step C.ii) is gradually pumped into the element used to stop the oxidation reaction. For example, the mixture obtained in step C.ii) is gradually pumped into deionized water to stop the reaction.

Optionally in step C.iv), graphite oxide is separated from the mixture obtained in step C.iii). Preferably, the graphite oxide is separated by centrifugation, by decantation or filtration.

Preferably, in step C.v), the exfoliation is performed by using ultrasound, mechanical agitator, sieve shaker or thermal exfoliation. Preferably, the mixture obtained in step C.iii) is exfoliated by using ultrasound into one or a few layers of graphene oxide.

Preferably, step D) comprises the following sub-steps:

-   -   i. the reduction of graphene oxide with a reducing agent,     -   ii. the agitation of the mixture obtained in step D.i),     -   iii. optionally, the washing of the reduced graphene oxide and     -   iv. optionally, the drying of the reduced graphene oxide.

In step D.i), preferably, the reducing agent is chosen from: acid ascorbic; urea; hydrazine hydrate; hydriodic acid; sulphur-based reducing agents such as sodium sulphite, sodium bisulfite, sodium thisulphate, sodium sulphide, thionyl choloride, sulphur dioxide; alkaline solution such as NaOH or KOH; phenols such as gallic acid, tannin acid, dopamine or tea polyphenol; alcohols such as methyl alcohol, ethyl alcohol or isopropyl alcohol; glycine; sodium citrate or sodium borohydride. More preferably, the reducing agent is acid ascorbic since the ascorbic acid is more environmentally friendly.

Advantageously, in step D.ii), the mixture is kept at a temperature between 50 and 120° C., more preferably between 60 and 95° C. and advantageously between 80 and 95° C. Preferably, the agitation is performed during less 24 hours, more preferably during less than 15 hours and advantageously during 1 to 10 hours.

By applying the method according of the present invention, reduced graphene oxide comprising below 20% by weight of oxygen functional groups and having an average lateral size below 30 μm preferably below 20 μm and more preferably below 10 μm comprising at least one layer sheet is obtained.

FIG. 1 illustrates an example of one layer of reduced graphene oxide according to the present invention. The lateral size means the highest length of the layer through the X axis, the thickness means the height of the layer through the Z axis and the width of the nanoplatelet is illustrated through the Y axis.

FIG. 2 illustrates an example of a few layers of reduced graphene oxide according to the present invention. The lateral size means the highest length of the layer through the X axis, the thickness means the height of the layer through the Z axis and the width of the nanoplatelet is illustrated through the Y axis.

Preferably, reduced graphene oxide is deposited on metallic substrate steel to improve some properties such as corrosion resistance of a metallic substrate.

In another preferred embodiment, reduced graphene oxide is used as cooling reagent. Indeed, reduced graphene oxide can be added to a cooling fluid. Preferably, the cooling fluid can be chosen from among: water, ethylene glycol, ethanol, oil, methanol, silicone, propylene glycol, alkylated aromatics, liquid Ga, liquid In, liquid Sn, potassium formate and a mixture thereof. In this embodiment, the cooling fluid be used to cool down a metallic substrate. For example, the metallic substrate is selected from among: aluminum, stainless steel, copper, iron, copper alloys, titanium, cobalt, metal composite, nickel.

The invention will now be explained in trials carried out for information only. They are not limiting.

Examples

Trials 1 and 2 were prepared by providing graphite electrode scrap from steelmaking plant. Trials 1 and 2 were respectively ground to have a size less than 150 μm and less than 450 μm.

After, Trials 1 and 2 were mixed with sodium nitrate and sulfuric acid. Potassium permanganate was slowly added into Trials 1 and 2. The mixture was kept at around 1° C. using a cooling system. Then, the cooling process was stopped and the mixtures were heated until reaching 35° C. to oxidize the graphite electrode scrap. After the oxidation, Trials 1 and 2 were gradually pumped into deionized water.

Then, the heat was removed and H₂O₂ in aqueous solution was added until there was no gas producing. MnO₂ was produced. HCl was added to the mixture to eliminate MnO₂.

Then, graphite oxide was washed and separated from the mixture by decantation. Then, it was exfoliated using ultrasound in order to obtain one or two layer(s) of graphene oxide.

Finally, graphene oxide of was separated from the mixture by centrifugation, washed with water and dried with air.

L-ascorbic acid was mixed with an aqueous solution of Trials 1 and 2. With L-ascorbic acid, some functional groups such as hydroxyl can be reduced but some groups such as epoxide groups cannot be reduced.

The reaction mixtures were agitated at 90° C. during 3 hours to reduce the graphene oxide sheets. Trials 1 and 2 were then washed and dried to obtain reduced graphene oxide powder.

Trial 3 is the disclosed Example prepared according to the method of the Korean patent KR101109961.

Results are shown in the following Table 1:

Method Trial 1* Trial 2* Trial 3 (KR101109961) Origin of raw material Graphite electrode scrap from Graphite electrode scrap Kish graphite from Steelmaking plant from Steelmaking plant Steel-mill byproduct Pre-treatment of raw Grinding Grinding to have a Grinding to have a size Flushing step Done with a solution material step size less than less than 450 μm of HCl and HNO₃ in 150 μm water Process of Done with a solution purification comprising EDTA using a salt, Na₂SO₃, Pretreatment surfactant, anionic, composition step nonionic polymer dispersant and distilled water Mechanical Done, Kish graphite separation step having a size below or equal to 40 mesh, i.e. 420 μm, kept Pre-treatment time 60 minutes 60 minutes >24 hours Pre-treated raw material >99% >99% At least 90% purity Oxidation preparation Done with H₂SO₄ and NaNO₃ Done with H₂SO₄ and Done with H₂SO₄ and NaNO₃ step of the NaNO₃ mixture Addition of KMnO₄ KMnO₄ KMnO₄ an oxidizing agent Element to Water followed by H₂O₂ Water followed by H₂O₂ Water followed by H₂O₂ stop the reaction Exfoliation Ultrasound Ultrasound Ultrasound Oxidation time 3 hours 3 hours 30 minutes Graphene oxide Graphene oxide comprising 48% Graphene oxide Graphene oxide having an average size of oxygen groups having an comprising 35% of oxygen between 12 and 20.5 μm and an average average Lateral size up to 10 μm groups having an average thickness between 5 and 120 nm with purity of 99.5% Lateral size up to 10 μm with purity of 99.5% Reduction step Done with acid ascorbic during 3 Done with acid ascorbic Done with hydrazine (N₂H₂) hours during 3 hours during 24 hours Reduced graphene oxide Reduced Graphene oxide Reduced Graphene oxide Reduced Graphene oxide having an comprising 15-20% of epoxides comprising 15-20% of average size between 12-25 μm and an groups and having an average epoxides groups and average thickness of 2-120 nm Lateral size from 5-10 μm, an having an average Lateral average thickness of 2- size from 5-10 μm, an 120 nm and a purity of 99% average thickness of 2- 120 nm and a purity of 99% *according to the present invention

The method of Trials 1 and 2 is more environmentally friendly than the method used for Trial 3. Moreover, the treatment of the raw material is performed very quickly compared to Trial 3. Finally, the reduced graphene oxide obtained with Trials 1 and 2 has a high purity and quality. 

What is claimed is: 1-13. (canceled) 14: A method for manufacture of reduced graphene oxide from graphite electrode scrap comprising: A. providing the graphite electrode scrap, B. grinding the electrode graphite scrap to obtain ground graphite electrode, C. oxidizing the ground graphite electrode to obtain graphene oxide including the following successive sub-steps: i. preparing a mixture including the ground graphite electrode, an acid and a nitrate salt, the mixture being kept at a temperature below 5° C., ii. adding an oxidizing agent into the mixture obtained in step C.i) to obtain graphite oxide in an oxidiation reaction, iii. after a targeted level of oxidation is reached, adding a chemical element to stop the oxidation reaction, iv. separating of the graphite oxide from the mixture obtained in step C.iii), v. exfoliating the graphite oxide into graphene oxide and D. reducing the graphene oxide into reduced graphene oxide. 15: The method as recited in claim 14 wherein in step B) the ground graphite electrode has a size less than 200 μm. 16: The method as recited in claim 15 wherein in step B) the ground graphite electrode has a size less than 150 μm. 17: The method as recited in claim 16 wherein in step B) the ground graphite electrode has a size between 100 and 150 μm. 18: The method as recited in claim 14 wherein in step C.i) the salt nitrate is chosen from at least one of the group consisting of: NaNO₃, NH₄NO₃, KNO₃, Ni(NO₃)₂, Cu(NO₃)₂, Zn(NO₃)₂, Al(NO₃)₃ and a mixture thereof. 19: The method as recited in claim 14 wherein in step C.i) the acid is chosen from at least one of the group consisting of: H₂SO₄, HCl, HNO₃, H₃PO₄, C₂H₂Cl₂O₂ (dichloroacetic acid), HSO₂OH (alkylsulfonic acid) and a mixture thereof. 20: The method as recited in claim 14 wherein in step C.ii) the oxidizing agent is chosen from at least one of the group consisting of: potassium permanganate, H₂O₂, O₃, H₂S₂O₈, H₂SO₅, KNO₃, NaClO and a mixture thereof. 21: The method as recited in claim 14 wherein in step C.iii) the element used to stop the oxidation reaction is chosen from at least one of the group consisting of: an acid, non-deionized water, deionized water, H₂O₂ and a mixture thereof. 22: The method as recited in claim 14 wherein a further chemical also is chosen to stop the reaction, the chemical element and the further chemical elements being used successively. 23: The method as recited in claim 14 wherein a further chemical also is chosen to stop the reaction, the chemical element and the further chemical elements being used simultaneously. 24: The method as recited in claim 14 wherein in step C.iii) the mixture obtained in step C.ii) is gradually pumped into the chemical element used to stop the oxidation reaction. 25: The method as recited in claim 14 wherein in step C.iv) the graphite oxide is separated by centrifugation, decantation, distillation or filtration. 26: The method as recited in claim 14 wherein in step C.v) the exfoliation is performed by using ultrasound, mechanical agitator, sieve shaker or thermal exfoliation. 27: The method as recited in claim 14 wherein step D) comprises the following sub-steps: i. reducing graphene oxide with a reducing agent, ii. agitating of the mixture obtained in step D.i), iii. optionally, washing the reduced graphene oxide and iv. optionally, drying the reduced graphene oxide. 